1. Field of the Invention
This invention relates generally to electric motors, and more particularly to unit bearing, fractional horsepower motors having an unusually small air gap.
2. Description of the Prior Art
Unit or single-bearing, shaded pole, single phase, alternating current motors are well known and are commonly used in air moving applications, a typical unit bearing motor construction being shown in U.S. Pat. No. 3,293,729 to the present applicant. Such conventional fractional horsepower, unit bearing motors have incorporated air gaps of about 0.009 or 0.010 inch, or more; to the best of the present applicant's knowledge, air gaps less than 0.008 inch have not heretofore been employed in such motors.
It has long been recognized that in the absence of losses, a decrease in air gap width with corresponding decrease in reluctance of the magnetic circuit will result in increased motor performance and efficiency. Resistance primarily limits the output in small shaded pole motors and thus, since the predominate current in such motors is magnetizing current, a decrease in the air gap width will result in a corresponding decrease in the input current and a proportionately greater decrease in copper loss (I.sup.2 R) thus, copper (or aluminum, as the case may be) and iron can be reduced, with consequent saving in cost and reduction in motor size, to bring the loss back to its original value, or alternatively with the same copper and iron configuration, locked rotor current and torque are increased to provide a "stiffer" motor.
While the potential advantages of reduced air gaps have been appreciated, the present applicant, and to the best of his knowledge other motor engineers, predicted that further reduction in air gap width beyond the widths currently employed would cause greatly increased pole face and tooth pulsation losses which would outweigh the advantages of the smaller excitation required by the smaller air gap. More particularly, as the air gap is made smaller, stator pole face and rotor tooth face losses increase due to the presence in the pole and tooth faces of magnetic flux of high space frequency which produces eddy-current and hysteresis losses of shallow penetration. It has been found that the magnetic flux from the rotor has ripples or harmonics creating 50 to 100 times as many poles as stator poles and further, the stator flux itself produces ripples or harmonics in the stator teeth.
I have found, most unexpectedly and surprisingly, that the use of extremely small air gaps, i.e., down to 0.001 inch, does not result in the aggravated tooth face and pole face losses I had predicted, while on the contrary that these previously unheard of small air gaps provide a very great improvement in performance and efficiency and thus afford a substantial reduction in overall cost.
I am aware that a miniature, two-bearing motor and motor alternator set was at one time provided for aircraft autopilot application. That instrument-type machine employed miniature ball bearings and was constructed with "white room" techniques to exceedingly accurate tolerances. The rotor members of those machines had an outside diameter on the order of one quarter inch with an air gap not less than 0.005 inch; however, the ratio of air gap width to rotor diameter was at least 0.020.